1. Field of the Invention
The present invention concerns methods and devices for correcting four field inhomogeneities in the basic magnetic field (B0) field in magnetic resonance imaging systems, particularly in such imaging systems using a multichannel reception and/or transmission RF (radio-frequency) coil configuration.
2. Description of the Prior Art
Functional magnetic resonance imaging (fMRI) is implemented by repeatedly acquiring image volumes, particularly of the brain, with fast imaging sequences such as echo-planar imaging (EPI) or spiral imaging. A sequence known as BOLD (Blood Oxygen Level Detection) generates a so-called BOLD signal that changes dependent on the oxygen level in blood in the detected region, this oxygen level, in turn being indicative of brain activity in the region. The detection of changes in the BOLD signal, which includes changes in Two*, CBF and CBV, relies on a statistical analysis of the time series implemented for each voxel in the examination region. For example, correlation analysis or General Linear Model (GLM) analysis of the time series with the paradigm that is employed (e.g., active and baseline conditions) convoluted with a hemodynamic response function is a basic procedure for fMRI. The underlying assumption is the stability of the measured signal in the spatial domain as well as in the temporal domain. Such preconditions are valid as well for other fast imaging applications, such as diffusion imaging and perfusion imaging.
Spatial stability can be disturbed by gross subject motion, and many techniques are known for correcting for spatial instability.
Temporal stability is maintained by fitting theoretical model functions of drifts (e.g., sine expansion terms) to the voxel-based time courses. Global frequency changes in the magnetic resonance system (Dynamic Off-Resonance changes in K-space, DORK) also can be monitored and corrected. Physiological signals such as respiration and heartbeat can be recorded by extracorporeal devices, and used as regressors in the fMRI analysis, such as in the GLM statistics.
Static and dynamic image deterioration in EPI images and spiral images has its source in a non-uniform B0 field, typically caused by the object itself brought into the B0 field (static B0 field distortion) or by external dynamic changes (dynamic B0 field change) caused by physiological changes, movement of the object, or system instabilities.
Mapping of the B0 field is a known technique resulting in B0 field maps, which are translated into image distortion maps or voxel displacement maps that are used for image correction in the image processing procedures that are employed.
The measurement of a high-quality B0 field map, however, requires at least multiple scan repetitions (TRs) lasting between 6 and 10 seconds a piece, and often resulting in a mapping procedure lasting between 1 and 2 minutes. These additional sources for dynamic noise and image deterioration hamper the detection of reliable MR images (fMRI, DWI, perfusion) and therefore it is essential to monitor such sources and to take their effect into consideration in the image correction, for example in the statistical analysis of and fMRI series. These noise sources exist in real time, and can be directly used for image correction during the progress of the data acquisition (in-line). Most often, however, more complicated statistical analysis is implemented after the measurement (data acquisition) has been completed (off-line post-processing).
Examples of known procedures for correction of temporal change due to physiological noise are described in “Retrospective Estimation and Correction of Physiological Artifacts in fMRI by Direct Extraction of Physiological Activity from MR Data,” Le et al., Magnetic Resonance in Medicine, Vol. 35, No. 3 (1996) pgs 290-298; “image-Based Method for Retrospective Correction of Physiological Motion Artifacts in fMRI: RETROICOR,” Glover et al., Magnetic Resonance in Medicine, Vol. 44, No. 1 (2000) pgs 162-167; “Functional MR Imaging in the Awake Monkey: Effects of Motion on Dynamic Off-Resonance and Processing Strategies,” Pfeuffer et al., Magnetic Resonance in Medicine, Vol. 25, No. 6 (2007) pgs 869-882 and “Correction of Physiologically Induced Global Off-Resonance Effects in Dynamic Echo-Planar and Spiral Functional Imaging,” Pfeuffer et al., Magnetic Resonance in Medicine, Vol. 47 (2002) pgs 344-353. EPI distortion correction (static and dynamic) is described in “Real-Time Autoshimming for Echo Planar Timecourse Imaging,” Ward et al. Magnetic Resonance in Medicine, Vol. 48 (2002) pgs 771-780; “Point Spread Function Mapping with Parallel Imaging Techniques and High Acceleration Factors: Fast, Robust, and Flexible Method for Echo-Planar Imaging Distortion Correction, Zaitsev et al., Magnetic Resonance in Medicine, Vol. 52 (2004) pgs 1156-1166 and Correction for Geometric Distortion and N/2 Ghosting in EPI by Phase Labeling for Additional Coordinate Encoding (PLACE),” Xiang et al. Magnetic Resonance in Medicine, Vol. 57 (2007) pgs 731-741.